Process for charging electrophotographic materials

ABSTRACT

A process for charging electrophotographic material having a photoconductive insulating layer on a high insulating support which is characterized by simultaneously applying high voltages of opposite polarities respectively to a first corona discharge electrode facing the surface to be charged of the photoconductive insulating layer and second corona discharge electrode or electrodes facing a surface of the photoconductive insulating layer to cause corona discharges of the opposite polarities to be applied to the photoconductive insulating layer and, no later than the corona discharge, uniformly irradiating the electrophotographic material through the support by a light which is absorbed by the photoconductive insulating layer and does not reach the surface of the photoconductive insulating layer.

nite Sato et al.

States Patent 1191 Jan. 29, 1974 1,963,615 7/1970 Germany 250/495 [75] Inventors: Masamichi Sato; Satoru Honjo; Primary ExaminerWilliam F. Lindquist Takao Komaki, all of Asaka, Japan Attorney, Agent, or FirmGerald J. Ferguson, .lr.; .10- 73 Assignee: Fuji Photo Film C0., Ltd., seph Bake Kanagawa, Japan [22] Filed: Sept. 17, 1971 57 ABSTRACT [21] Appl' lslszl A process for charging electrophotographic material having a photoconductive insulating layer on a high [30] Foreign Application Priority Data insulating support which is characterized by simulta- Sept. 18, 1970 Japan 45/81731 neously pp y high voltages of pp polarities respectively to a first corona discharge electrode fac- 52 U.S. c1. 250/325, 317/262 A ing the surface to be charged ef the pheteeenduetive [51] Int. Cl G03g 13/02 insulating layer and Second corona discharge elec- [58] Field f 25 /495 QC, 495 ZC, 495 trode or electrodes facing a surface of the photocon- 317 2 2 9 R 1 C ductive insulating layer to cause corona discharges of the opposite polarities to be applied to the photocon- 5 References Cited ductive insulating layer and, no later than the corona UNITED STATES PATENTS discharge, uniformly irradiating the electrophoto- I graphic material through the support by a light which ii fii is absorbed by the photoconductive insulating layer 4 5 09 7/969 :35 M9 5 and does not reach the surface of the photoconductive insulating layer- FOREIGN PATENTS OR APPLICATIONS 971,281 9/1964 Great Britain 250/49.5 7 Claims, 5 Drawing Figures 34 i '1 3O 34 ['1 /l' l V/ I 117- J v 35 g u p 35 f ,1

PROCESS FOR CHARGING ELECTROPHOTOGRAPHIC MATERIALS BACKGROUND OF THE INVENTION This invention relates to a process for charging a photoconductive insulating layer formed on a high insulating support.

Electrophotographic material having a photoconductive layer on a support of relatively high electric resistance material such as an ordinary paper, can be charged in a so-called double corona charging process. However, it is difficult to charge an electrophotographic material with a support of a very high electric resistance material. The support of a very high resistivity may be exemplified by those formed of polyethylene, terephthalate, polyethylene, polypropylene, polyvinyl chloride or triacetylcellulose. Accordingly, such electrophotograhic materials are not usually put to use, and, in cases of ordinary electrophotographic materials having such a. support, there is formed an electroconductive layer on such an insulating support and thereon a photoconductive insulating layer.

SUMMARY OF THE INVENTION The present invention provides an effective charging process for electrophotographic materials of the type having a photoconductive insulating layer directly on a highly insulating support. This is effected by providing a process for charging electrophotographic member consisting of a photoconductive insulating layer having predetermined majority and minority charge-carriers on a high insulating support which is characterized by simultaneously applying high voltages of opposite polarities respectively to a first corona discharge electrode facing the surface to be charged of the photoconductive insulating layer and at least one second corona discharge electrode facing a surface of the photoconductive insulating layer to cause corona discharges of the opposite polarities to be applied to the photoconductive insulating layer so that the polarity of the corona discharge applied by the first corona discharge electrode is the same as that of the predetermined majority carriers and, no later than the corona discharge, uniformly irradiating the exterior surface of the high insulating support of the electrophotographic material through the high insulating support by a light which is absorbed by the photoconductive insulating layer and does not reach the surface to be charged of the photoconductive insulating layer so that an electrically conductive region is established in the photoconductive insulating layer adjacent the high insulating support to facilitate the flow of the predetermined majority charge carriers from the electrically conductive region to the corona discharge applied by the second corona discharge electrode whereby the minority charge carriers remain in the conductive region to facilitate the charging of the photoconductive insulating layer with the corona discharge from the first corona discharge electrode.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is an enlarged cross-sectional view of an electrophotographic material used in the present invention.

FIG. 2 is a schematic side view of an apparatus for carrying out a charging process.

FIG. 3 is a schematic side view of an apparatus for carrying out the charging process of the present invention.

FIG. 4 is an enlarged from view for illustrating the principle of the present invention, and FIG. 5 is a schematic view of another apparatus for carrying out the process of the present invention.

DETAILED EXPLANATION OF THE INVENTION FIG. 1 is a cross-sectional view of an electrophotographic material 10 used in the present invention, in which numeral 11 indicates a photoconductive layer formed of e.g., vacuum evaporated amorphous selenium, a mixture of an insulating organic resin and a powdered photoconductor such as, e.g., zinc oxide or cadmium sulfide, or an organic photoconductor, and numeral 12 indicates a highly insulating support such as well dried paper or film of polyethylene terephthalate, polyethylene, polypropylene, polycarbonates, polyvinyl chloride or triacetylcellulose.

FIG. 2 illustrates a known process for charging an electrophotographic material. The charging process as illustrated by FIG. 2 is disclosed in British Patent Specification 971,281. In this process, electrophotographic material 10 is used in the form of a elongated film wound on feed roll 20. Numeral 21 indicates a contact piece, 22 indicates a light source irradiating the back surface of the electrophotographic material, 23 indicates a corona electrode and 24 indicates a shield case.

The light source 22 emits such a light that is transparent to the support of the photoconductive layer but adsorbed by the photoconductive layer 11. When irradiated by such a light source, the back surface of the photoconductive layer is rendered electroconductive, and is chargeable by subjecting it to a corona discharge while it retains the conductivity. For charging the electrophotographic material by the corona discharge it is necessary to earth the electroconductive layer, and it is accomplished, in the case of an apparatus as illustrated in FIG. 2, by contacting the contact piece 21 with the surface of the photoconductive layer.

However, the contact with the contact piece 21 with the surface of the photoconductive layer is apt to form streaky scratches on the surface of the photoconductive layer. Moreover, it is difficult to attain complete contact of the contact piece with the surface of the photoconductive layer, so that it is difficult to attain a uniform charging thereby. It is necessary for attaining good contact to strongly force the contact piece to the surface of the photoconductive insulating layer, but this causes more heavy streaky scratches.

The present invention provides a charging process free from such drawbacks.

FIG. 3 is a schematic front view of an apparatus for carrying out the process of the present invention. Numeral 30 indicates a main discharge unit comprising a corona wire 31, a shield case 32 and insulators 33 supporting the corona wire. Numeral 34 indicates subsidiary charging units on each side of the main charging units on each side of the main charging unit 30 each consisting of a corona wire 35 and a shield case 36. The wire 31 and 35 meet at right angle with each other, and, in FIG. 3, the wire 31 is parallel to the plane of this paper and the wire 35 is perpendicular to the plane of this paper. The charging may be carried out by moving the main charging unit in the direction perpendicular to the plane of this paper, or, alternatively, by moving the electrophotographic material in the direction perpendicular to the plane of this paper while keeping the discharge unit to stand still. In case where it is desired to charge in minus the surface to be charged, there are applied a minus high voltage to the corona wire 31 of the main charging unit and a plus high voltage to the corona wire 35 of the subsidiary charging unit 34 to cause corona discharge. On this occasion, the electrophotographic material is uniformly irradiated from the back thereof by a light which will be adsorbed by the photoconductive insulating layer from a light source 37. In case where it is desired to charge in plus the surface to be charged, the polarities of the voltages applied to the both wires are reversed. As the light source suitably used are those emitting such lights which will be well absorbed by the photoconductive insulating layer and render the photoconductive insulating layer conductive. For photoconductive insulating layers formed of selenium or an organic photoconductor there are used light sources emitting blue light to ultraviolet rays, and for a photoconductive insulating layer formed of a mixture of powdered zinc oxide and a resin there is used a light source emitting ultraviolet rays.

The principle of the present invention will be illustrated by FIG. 4. In the FIG. 4, the photoconductive insulating layer is formed of a N-type semiconductor, such as a mixture of a photoconductive powdered zinc oxide and an insulating resin. Numeral indicates a main discharging unit, 41 indicates a subsidiary discharge unit and 42, 43 indicate corona wires in these units. When a plus high voltage, e.g., of +6 KV, is applied to the corona wire 43 and a minus high voltage, 0g. of 6 KV is to the corona wire 42, plus corona ions is emitted from the wire 43 toward the photoconductive layer and deposit on the surface of the latter. Since the photoconductive insulating layer 11 is of N-type and allows electrons to move freely therein, free electrons indicated by@ come from the electrpconductive layer 44 which has been formed in the back surface of the photoconductive layer by the irradiation by light and neutralize the plus charges given by the plus corona ions. Consequently, there remain in the electroconductive layer 44 plus charges indicated byB which facilitate supply of minus corona ions from the corona wire 42 to the surface of the photoconductive layer. That is to say, there is obtained an earth effect. According to our experiments, supposing that the amount of electric charge supplied from the corona wire 42 was l when no voltage was imposed to the corona wire 43, the amount of electric charge increased to 100 to 800 when to the corona wire 43 was applied a voltage of the opposite polarity but the same absolute value to the voltage applied to the corona wire 42.

There may be used a ribbon or needles in place of a corona wire as a corona discharge electrode.

In an embodiment illustrated by FIG. 3, there are two subsidiary discharge units, though necessary is at least one subsidiary discharge unit and, if desired, there may be used three of more subsidiary discharge units.

It is disadvantageous that corona ions from a main discharge electrode and corona ions from a subsidiary discharge electrode overlap on the surface to be charged, so that it is preferred to put a partition plate between the two discharge electrodes. The partition plate may be served by the shield of one discharge electrode.

FIG. 5 is a schematic cross-sectional view illustrating another embodiment of the present invention. In the FIG. 5, an electrophotographic material is uniformly exposed from its back to light, then irradiated by plus corona ions by means of a first corona charging unit 50 and subsequently by minus corona ions by means of a second corona charging unit 51 and, thereby, charged with minus electric charges. Uniform charging throughout the whole surface of the electrophotographic material is attained by use of corona wires of a length larger than the width of the electrophotographic material.

Although there is no special problem in case where exposure to light from the back of an electrophotographic material is carried out concurrently with charging by corona discharge, it is necessary, in case where exposure to light is performed prior to charging, for assuring a sufficient electroconductivity to remain in the back surface of the photoconductive insulating layer during charging to appropriately select the amount of exposure, time and characteristics of photosensitive layer.

The present invention is illustrated in more detail by the following examples. As mentioned above, it is necessary to use a light within a spectral range which would be strongly absorbed by the photoconductive insulating layer.

For instance, in the following Example 1 and Example 2, there was used a photosensitive insulating layer containing a zinc oxide which had not been dyesensitized and there was used a tungsten lamp for irradiation of the back of the photosensitive insulating layer.

Since the photoconductive insulating layer has a photoconductive sensitivity to ultraviolet to near ultraviolet rays only and strongly absorbs the rays within the spectral ranges, among light to which the photoconductive insulating layer is exposed availed are ultraviolet and near ultraviolet rays only. And, since the rays are absorbed by a very thin surface layer of the back of the photosensitive insulating layer, they have little effect on the surface layer of the photoconductive insulating layer.

In case of a photoconductive insulating layer containing a dye-sensitized zinc oxide, the spectral sensitive region is extended from near ultraviolet to visible regions and the photosensitive insulating layer exhibits a low absorption factor and a high photosensitivity to light of visible region. Accordingly, in cases where portions of visible rays irradiated from the back of a photoconductive insulating layer reaches the surface thereof, the charging property of the photoconductive insulating layer is lowered. On such occasions, it is desired to use a filter which absorbs lights in visible region but transmits ultraviolet rays.

Example I.

An electrophotographic material was prepared by coating on a I50 microns thick poly(ethylene terephthalate) film a mixture of parts by weight of a photoconductive powdered zinc oxide with 20 parts by weight of an insulating resin, styrenated alkyd resin supplied by Japan Reichhold Co., under a trade name of Styresol 4400, to form a coating film of a dry thickness of about 7 microns.

The electrophotographic material was put on a high insulating bed (10 mm thick poly(methyl methacrylate) sheet) in a dark place and a corona discharge unit as shown in FIG. 3 was put thereover. The main discharge unit had a distance between corona wire and the surface to be charged of 15 mm; a distance between the corona wire and shield case of 15 mm; the diameter of the corona wire of 0.05 mm; the length of the corona wire of 300 mm; and the distance between the lower edges of the shield case and the surface to be charged of 8 mm. The corona wire was formed of tungsten. The subsidiary discharge unit had: a distance between the corona wire and the surface to be charged of mm; a distance between the shield case and corona wire of mm; a diameter of the wire of 0.05 mm; a length of the wire of 50 cm; and a distance between the lower edges of the shield case and the surface to be charged of 2 mm. The corona wire also was formed of tungsten.

The electrophotographic material was relatively moved at a rate of 50 mm/sec in the direction perpendicular to the main discharge electrode while applying a voltage of 6 KV to the main discharge electrode and voltage of +7 KV to the subsidiary discharge electrode. The electrophotographic material was uniformly charged at a surface potential of l80 V except the both edge portions. To say, the areas under the subsidiary discharge electrodes were not charged.

The electrophotographic material had been uniformly irradiated from its back by means of a tungsten lamp. The amount of exposure was 2,000 lux. 2 sec. The charging was completed within 30 seconds after the exposure.

Example 2.

An electrophotographic material as in Example 1 was put over an insulating sheet as in Example 1, and two needle electrodes were set thereover perpendicularly to the surface to be charged. The distance between the two corona discharge electrodes was 180 mm. One of the corona discharge electrode (main discharge electrode) was positioned over the center of the surface to be charged and the others (subsidiary electrodes) were positioned over the edges of the surface to be charged. The distance between the tip of the main electrode and the surface to be charged was 70 mm and that between the tip of the subsidiary electrode and the surface to be charged was 10 mm. To the main discharge electrode was applied a voltage of 10 KV and to the subsidiary discharge electrode was applied a voltage of +40 KV. When the electrophotographic material was moved at a rate of 30 mm/sec under the electrodes, the center portion of the surface to be charged was charged at a surface potential of 1 50 V. As in Example 1, the electrophotographic material was uniformly exposed from its back to light prior to charging. The amount of exposure was 1,500 lux 15 sec. and the charging was completed within 30 seconds after the exposure.

Example 3.

The procedure as in Example 1 was repeated except that the exposure to light was performed concurrently with the corona discharge. To say, the electrophotographic material was subjected to corona discharge while exposing it to a uniform exposure to light of 4,000 lux. sec from its back. Then, the photoconductive insulating layer was charged to a surface potential of l80 V.

Example 4.

The material as in Example 1 was wound into a continuous roll 20, as shown in FIG. 5, and withdrawn at a rate of 50 mm/sec under a light source and charging units positioned as shown in FIG. 5. The light source was consisting of 6 tungsten lamps each of 300 watts in 2 rows as to provide an amount of exposure for the electrophotographic material passing thereover of 8,000 lux. sec. The electrophotographic material passed over the light sources was passed over a first charging unit and then a second charging unit. in the both charging units, the corona wires were of stainless steel and of a diameter of 0.05 mm, the distance between the shield case and wire was 15 mm and the distance between the wire and surface to be charged was 12 mm. The distance between the wire in the first charging unit and that in the second one was mm. When to the wire in the first charging unit was applied a voltage of +7.5 KV and to the wire in the second charging unit was applied a voltage of 7.5 KV, the electrophotographic material was charged to a surface potential of l20 V. The length of the corona wire was about 50 mm larger than the width of the electrophotographic material.

What is claimed is:

ll. A process for charging electrophotographic member consisting of a photoconductive insulating layer having predetermined majority and minority charge carriers on a high insulating support which is characterized by simultaneously applying high voltages of opposite polarities respectively to a first corona discharge electrode facing a portion of the surface to be charged of the photoconductive insulating layer and at least one second corona discharge electrode facing another portion of the exterior surface of said photoconductive insulating layer to cause corona discharges of said opposite polarities to be applied to different portions of said photoconductive insulating layer so that the polarity of the corona discharge applied by said first corona discharge electrode is the same as that of said predetermined majority carriers and, no later than said corona discharge, uniformly irradiating the interior surface of said photoconductive insulating layer of said electrophotographic material through said high insulating support by a light which is absorbed by said photoconductive insulating layer but does not reach the surface to be charged of the photoconductive insulating layer so that an electrically conductive region is established in said photoconductive insulating layer adjacent the high insulating support to facilitate the flow of said predetermined majority charge carriers from said electrically conductive region to the corona discharge applied by said second corona discharge electrode whereby said minority charge carriers remain in said conductive region to facilitate the charging of said photoconductive insulating layer with the corona discharge from said first corona discharge electrode.

2. A process as in claim 11 where said photoconductive insulating layer is N-type.

3. A process as in claim 1 where said photoconductive insulating layer is P-type.

4. A process as in claim l where said second corona discharge electrode faces the surface to be charged of the photoconductive insulating layer.

5. A process as in claim 4 where said first corona discharge electrode faces the central portion of said surface to be charged and said second corona discharge electrode faces the portion adjacent the edge of said surface to be charged.

6. A process as in claim 4! where said first and second corona discharge devices extend across said surface to be charged, said devices being laterally displaced from one another.

7. A process as in claim l where said second corona discharge electrode faces the edge portion of said photoconductive insulating layer. 

2. A process as in claim 1 where said photoconductive insulating layer is N-type.
 3. A process as in claim 1 where said photoconductive insulating layer is P-type.
 4. A process as in claim 1 where said second corona discharge electrode faces the surface to be charged of the photoconductive insulating layer.
 5. A process as in claim 4 where said first corona discharge electrode faces the central portion of said surface to be charged and said second corona discharge electrode faces the portion adjacent the edge of said surface to be charged.
 6. A process as in claim 4 where said first and second corona discharge devices extend across said surface to be charged, said devices being laterally displaced from one another.
 7. A process as in claim 1 where said second corona discharge electrode faces the edge portion of said photoconductive insulating layer. 